The invention relates to a temperature monitoring system for use with a rotary thermoforming machine. In a particular example it relates to use of an infra-red linescanner to obtain temperature images of plastic sheets on the “carousel” of a rotary thermal forming machine.
In typical thermal forming processes, plastic sheets are heated prior to being formed into various articles. Examples of such articles include baths, refrigerator liners, car dashboards and yoghurt cartons.
Heating softens the plastics material, and is usually carried out by placing the plastic sheet underneath a heater panel or between two such panels. Forming is then usually carried out by “sucking” the softened sheet into a mould. Typically this is achieved by creating a vacuum or low pressure in the mould cavity and allowing the softened plastic to be pressed into the mould by the action of atmospheric or applied pressure.
Depending on the application, the sheets can be large. For example, each sheet may be up to several square meters in area.
Often, rather than heat the entire plastic sheet to a uniform temperature, it is desirable to establish a temperature pattern in which the temperature varies across the sheet in a controlled manner. For instance, higher or lower temperatures (i.e. softer or harder plastic) may be required in different regions of the sheet depending on the amount of deformation those regions will experience in the forming process.
The temperature of the heater panel is controlled in a large number of “zones”. This is done by conventional control of numerous electrical resistance heaters to temperature set-points.
In linear thermoforming machines, the sheets move in a straight line throughout the process. In rotary thermoforming, machine sheets are attached to a revolving “carousel” and move in a circle or along a circular arc.
For accurate control of the thermoforming apparatus, it is useful to obtain a temperature image of the plastic sheet just after heating—e.g. as it leaves the heater panel. Comparison of this image with an “ideal” image allows the heater panel set-points to be optimized.
Specifically it is desirable to obtain a temperature “map” referenced to, and correctly dimensioned with respect to, the (usually rectangular) sheet. This temperature map can then be directly related to the heater pattern. For example specific areas on the map may be identified as corresponding to specific heaters (or groups of heaters operated from a single controller) in the heating panel. It is further possible to over-lay the temperature map on to a drawing of the thermoformed sheet—this allows the temperature pattern to be related directly to the deformation pattern.
A temperature image of the sheet may be obtained using an infra-red linescanner. The sheet moves substantially horizontally and the linescanner is aimed substantially vertically with its scan plane transverse to the direction of sheet motion. The 1-dimensional scan plus the sheet's linear motion allows a 2-dimensional image to be captured.
For example, a typical infra-red linescanner may capture 1000 temperature points along each scan line and make 100 scans per second. Thus, if the sheet takes 2 seconds to traverse the scan plane, a 1000 by 200 pixel temperature image results.
Typically the linescanner uses an opto-mechanical scanning device. This rotates a measurement axis, aligned to the infra-red beam, at fixed angular velocity within a scan plane. The centre of rotation is a rotation axis, perpendicular to the scan plane, located inside the linescanner sensing head. The instantaneous measurement point on the sheet is the intersection of the measuring axis with the plastic sheet.
Before discussing the problems encountered with linescanning on rotary machines it is useful to consider the simpler situation of a linear machine.
Here the sheet moves horizontally in a straight (rectilinear) line. We will assume, for the moment, that the sheet is rectangular and moves at constant speed in a direction parallel to its length.
The linescanner is typically fixed above the centre-line of the sheet and adjusted so that the scan plane is vertical and perpendicular to the sheet's motion.
The linescanner generates a temperature signal according to the instantaneous position of its measurement axis.
The linescanner also generates a “scan valid” signal. This is a rectangular waveform whose rising and falling edges mark the beginning and end of valid measurements for each scan. Typically these correspond to the measurement axis being 40 degrees either side of “scan-centre”. Typically scan-centre is perpendicular to the front face of the scanning instrument and, in this arrangement, to the plane of the sheet.
The two signals are processed by a computer or processor. The temperature signal is sampled at uniform time intervals. Data is recorded only when the scan valid signal is “high” and is organized into a series of “scan lines”, a new line being started each time the scan valid signal transitions from low to high.
Temperature samples within each scan line are numbered.
Scan lines are also numbered, e.g. starting from when temperatures above a pre-set threshold are detected—for instance when the sheet starts to cross the scan plane. Alternatively, the start of count may be triggered by an external digital signal which flags arrival of the target sheet.
The data obtained from the linescanner is processed in a Cartesian array of samples and the temperature image is displayed on the computer screen as a Cartesian array of pixels whose color represents temperature according to a pre-defined “palette”. In this array, the ordinate (i.e. row) corresponds to the sample number and the abscissa (i.e. column) corresponds to the scan line number. The image builds up approximately in real time as the sheet crosses the scan plane.
However, a temperature image constructed according to the above method is geometrically distorted with respect to the plastic sheet. The following factors contribute to this:    1. The temperature samples are taken at equal time intervals, which do not equate to equal spatial intervals across the width of the sheet. Specifically, there is a sec2θ variation where θ is the angle between the instantaneous position of the measurement axis and the vertical (perpendicular to the plane of the sheet).    2. The sheet moves throughout the time taken to make each scan line. Scan lines are therefore slightly oblique when referenced to the sheet, i.e. not completely parallel to the sheet edge.    3. The x axis is not correctly scaled relative to the y axis.
For many purposes these distortions are not serious. The image of a rectangular sheet moving at constant speed in a direction parallel to its length remains rectangular and it is easy to visually relate the temperature image to the sheet geometry and hence to the zone pattern on the heater panel.
If required, the distortions can be removed. Effect (2) is typically small and can be reduced arbitrarily by increasing the scan speed. Effect (3) is obviated by re-scaling the image to match known sheet dimensions. Effect (1) may be removed by additional processing as follows:
For each temperature sample, the instantaneous angle θ of the measurement axis to the vertical is calculated from the known sample number, sample interval and pre-set parameters in the linescanner.
For example, assuming that the scan valid signal rises 40 degrees before scan centre and falls 40 degrees after scan centre and that at scan centre the measurement axis is vertical. The measurement axis makes 100 full 360 degree rotations per second—i.e. the scanning line traverses the 80 degree measurement zone in 2.22 milliseconds. Temperature samples are taken every 2.22 microseconds (1000 samples per scan) following the rise of the scan valid signal. In this case, the angle θ for the nth sample is θ=40*(n/500−1).
The instantaneous position Y of the measurement point across the width of the sheet, referred to the scan-centre, is then calculated as Y=H*tan θ where H is the measured height of the linescanner above the sheet. This height measurement is made to the centre of rotation of the measurement axis (this position is fixed and known in relation to the linescanner housing).
Each temperature sample is thus labeled with a true position across the sheet.
The data is now plotted on the computer screen as a Cartesian array of pixels where the “Y” position (i.e. row) represents position-across-sheet, whereas the “X” position (i.e. column) is scan line number as before. The pixels are positioned at uniform Y increments which will not, in general, correspond exactly to measured sample positions. The appropriate temperature at a given pixel location is calculated by interpolating between adjacent samples or by assigning to the pixel the temperature of the nearest-located sample.
In some linear machines, the sheet velocity varies substantially while the sheet is under the scanner. This leads to a more serious distortion. Scan lines are now spaced non-uniformly along the sheet. A Cartesian temperature image with ordinate in scan line number “stretches” those parts of the sheet which move slower through the scan plane and “squashes” those parts which move faster.
This can be rectified only if a sheet position (or speed) signal is made available to the computer. This allows scans to be labeled with position-along-sheet rather than just scan number. A Cartesian temperature image can then be constructed where the abscissa is position-along-sheet.
In a rotary thermoforming machine, the sheet moves in a circle or arc rather than a straight line. Further, the speed is far from uniform. As a result, temperature images obtained using linescanners suffer massive geometrical distortion.
Typically up to four sheets are mounted horizontally on a carousel which rotates about a vertical axis. The carousel moves rapidly, with high acceleration and deceleration, through successive (on a four sheet machine) 90 degree movements. The following discussion focuses on a four sheet machine. However, the mechanism is similar for machines with any other number sheets.
Referenced to the ground, G, beneath the carousel there are four locations, referred to as; “load”, “heat”, “form” and “cool”.
The journey of an individual sheet is as follows:                Placed on the carousel at the “load” location        Moved to the “heat” position (between the heater panels) and held for specified time        Moved to the “form” position, sucked into the mould and subsequently expelled from it        Moved to the “cool” location to cool and subsequently be removed from the carousel.        
If an infrared linescanner is mounted so as to scan the sheets exiting the “heat” position and a Cartesian temperature image is plotted with sample number and scan number as ordinate and abscissa respectively (using the above described technique) then a grossly distorted temperature image is obtained.
The distortion is caused primarily by two effects:    1. The “outer” edge of the sheet traverses the scan plane faster than the “inner” edge. Thus, referred to the sheet, scan lines are further apart at the outer edge than at the inner edge. On the temperature image however scan lines are parallel. This is depicted schematically in FIGS. 6A and 6B where R–R′=represents the sheet's inner edge and Q–Q′=the outer edge.    2. Since the sheet accelerates and decelerates during rotation, the leading and trailing edges of the sheet traverse the scan plane slower than does the centre of the sheet. Thus, referred to the sheet, scan lines are further apart in the centre of the sheet than at the leading and trailing edges. On the temperature image, scan lines are equidistant.
It is very difficult to visually relate the resulting temperature images to the sheet or to the zone pattern of the heater panel.
One technique for reducing this distortion is disclosed in U.S. 2004/0251407. This system uses a zone calibrator sheet, prepared by the user, which has the position of the heating zones defined on its surface by a thermally contrasting material. The calibrator sheet is transported through the thermoforming apparatus, with or without heating, and imaged using a line scanner. The user then traces the position of each heating zone on the resulting distorted image to produce a grid image which is stored and used for comparison with thermal images of subsequent sheets.
This method suffers from a number of disadvantages. The use of a calibration object is expensive and time consuming, requiring a new calibration sheet for each new heating pattern or sheet size and shape. As a result, calibration is not performed in real time. The system requires a lot of user intervention, both in producing the calibration sheet and in interpreting its image so as to generate the grid. Each of these steps will introduce error. Further, the accuracy of the calibrated image is inherently low since the resolution is limited to zones and the system cannot compensate for unexpected variations in sheet position, velocity or acceleration.